- Email: [email protected]
A study of valence band shake-up features of CO on Pt {100} using synchrotron radiation
CHEWCAL
Volumr 1 cl@, number 4
A STUDY OF VALENCE
BAND SHAKE-UP
USING SYNCHROTRON
RADIATION
16 September 1983
PI1YSICS LETTERS
FEATURES
OF CO ON Pt
{loo}
D.E. GRIDER. KG. PURCELL and N.V. RICHARDSON i%e Donnan Laboratories. Um+ersit_v of Liverpool, Grove Street. PO Box 147. Liverpool L69 3BX. UK Recribed 11 April 1983: in final form Z July 1983
A stud! of the shAe-up fearures in rhc valence region of CO on Pt {loo) is presented. The onset of these features is shown IO be correlntcd wirh the excitation threshold for the I’t(4t-l levels. A possible mechanism for the shake-up process is proposed in uhich the Pt(Jfl hole results in exited skxtes other than a single electron-hole state. Subsequently, the Pt(lf) hole decals via a direct recombination process resultins in the observed satellite features. The implications of such a shakeup process for photon-srirnulatcd desorprion (PSD) are also discussed.
We report the first clear evidence for interatomic shake-up features associated with the valence levels of a cbemisorbed molecule_ The onset of shake-up structures in the valence region of CO/Pt { 100) is shown to be correlated with the Pt(4f) excitation threshold_ Such an onset indicates that the presence of this core-hole results in intermediate escited states other than a single electron-hole state. Subsequent decay of these states i;lvolving a direct recombination mechanism for the Pr(4f) hole results in the observed satellite features_ The investigation of satellite structures arising from photoexcitation
of electrons
from
core
levels has long
been an area
of intense interest in both gas-phase and surface-adsorbate studies [l-6] _ Recently, however, there has been increased interest in similar satellites asso&ted with valence levels or adsorbed molecules [6-IO]_ In a recent paper. Loubriel et al. [ 71, have observed valence shake-up states associated with the creation of a hole in the C( 1s) level of CO on Pt {I 11). These shake-up features were observable only over a very narrow photon energy range corresponding to the resonant photoescitation of an electron level of CO followed by 311 Auger decay process. The predominant features were associated with local ligand excitations (i.e. 5~~’ lsr-l 2~*l, 50-z %‘I, ln-2 %‘I. and 40-1 50-1 25r*1) as might be expected due to the presence of the C(ls) hole on
the CO molecule
itself. In addition,
however, less
intense lower binding energy satellites here observed which were associated with charge-transfer excitations (In-’ 5d-’ 2n*‘, and 5u-’ 5d-’ 2~*~). On the basis of this work, one might reasonably expect to observe analogous valence band satellite features upon creation of a core hole in the metal substrate followed by an Auger decay process. Indeed, recent photoemission studies of CO on Pt [lo] show a weak CO-induced shoulder =5 eV below the 40 peak over a photon energy range from 120 eV up to 190 eV. This feature, however. was tentatively ascribed to a 5d to 2n* excitation based on comparison with shake-up structures in the valence bands of transition-metal carbonyls [5,6] _ These structures are thought to correspond to screening of 4u and 50 valence holes by charge transfer of metal electrons to the In* orbital as well as local ligand excitations. Recent studies of CO on CU { 100) [9] have shown that screening of valence holes on CO by metal electrons can result in striking satellite features just as it does for core holes [4]. Strong satellites are observed ~1.7 eV below the 40 and %/In main-line peaks. These features were attributed to final-state configurations in which the 4u,5u. and 1s valence holes are screened (main-line peaks) or relatively unscreened (satellites) by the metal electrons. In addition, there is a very weak feature a5.5 eV below the 40 main-line peak which is believed to be due to intramolecular ligand excitations (i.e. Ix to 2n*).
320 0 009~2614/83/0000-0000/S
03.00 G 1983 North-Holland
Volume 100. number 4
CHEMICAL
PHYSICS LETTERS
16 September
1983
The purpose of this work is to find evidence for valence band shake-up features associated with the creation of a core hole in the metal substrate_ It should be possible to distinguish between such features and those due to screening of valence holes in the adsorbate by metal electrons or due to local ligand excitations simply by Iooking for the onset of satellites at a photon energy corresponding to the creation of a core hole in the metal. An estimate of the photon energy threshold of such an excitation involving a core hole of ionization potential (1r,) is given by hu,
=‘n
-am
+A,,
(1)
where @nr is the work function of the metal and As is the energy given over to the shake-up state as determined by the difference in the apparent binding energies between the CO main-line and shake-up peaks. This results in Jzvth =72+3eV
(2)
for a Pt(4f) excitation threshold and a shake-up energy (As) of 6 i- 3 eV_ The measurements were carried out at the Daresbury Synchrotron Radiation Source using a Vacuum Generator ADES_400 photoelectron spectrometer_ Photoelectrons were collected along the [ 1001 plane of incidence for a range of emission angles (8) and various angles of incidence (or) for p-polarized light. Following repeated argon-ion bombardment/anneal cycles, the surface cleanliness of the Pt {I 00) crystal was monitored using AES. In addition, a clear (5 X 20) LEED pattern was observed, indicative of the cIean reconstructed Pt { 100) surface 11 I] _For all of the measurements presented here, the sample was dosed with 4 L of CO at 300 K. The background pressure during the measurements was in the low 1O-lo mbar range. As shown in fig. la the photoemission spectra (Jzv= 65 eV) at photon energies below the threshold for the creation of a Pt(4f) core hole exhibits the normal COinduced photoemission peaks at binding energies of 8.8 eV (%/lrr), 11.3 eV (4~) and 32.0 eV (3~). In addition, there is a weak OW Auger feature at an apparent binding energy of 23 eV_ At a photon energy of 7 1 eV, as shown in fig. I b, additional CO-related features appear at one-electron binding energies of -14 and 17 eV. At a slightly higher photon ener,T of 77 eV, as shown in fig. 1c, the 17 eV feature is well resolved and there is still a broad shoulder in the region of 14 eV binding energy.
J, ?I$
1
5
1-
10
3tNDlNG
I
15
I
20
I
25
I
30
I
35
ENERGY/eV
Fig_ 1. Photoelectron spectra for 4 l_ of CO on IQ {OOI) at an ate of incidence (&I of 40’ and emission an& (6) of 35O as a function of photon energy: (a) hv = 65 eV; (b) hu = 71 eV; and (c) hv = 77 eV. The arrows mark the approximate positions of the satellite features. In curve (c), the feature aboveEfis due to excitation of the Pt(4f) levels by second-order light from the monochromator_
The onset of the 17 eV satellite structure at a photon energy of 71 eV is in excellent agreement with the estimate for JzvU, given by eq. (3) Although the 14 eV shoulder is not well resolved at all photon energies, it too has a threshold in the same photon energy region. This is a clear indication that both of these shake-up features are associated with the creation of a Pt(4f) core hole. Both of these structures remain at the same binding ener,v and have approximately the same intensities relative to the Su/ls and 40 main-line peaks up to a photon ener7 of at least 130 eV. As noted previously, similar structures have been observed up to =I90 eV
WIAlthough
a detailed
discussion
of the dependence
of
321
CIiE!MC_M_ PHYSICS LETTERS
Solurnc 1130.number 4
the sateIiire features on angle of incidence (CY)of the llgbt and photoelectron emission angle (0) will be presented in a later work 1 I?]. some general trends should be noted. As might be expected simply on the basis of electron escape depth arzunrents. both the 14 and 17 eV satellites increase as one moves away from normal emission (0 = 0). Xs shown in figs. ta and 2b. the dramatic increase in the 40 main-line feature (1 1.3 eV) between 0 = 0” and 0 = 60” for constant angle of in-
16
September 1983
cidence (a = 20”) is mirrored in both the 14 and 17 eV satellite structures. It is not, however, possible to eliiinate completely the possibility that excitations of 50 or 1s levels are also involved in these satellite structures since the So/lx n&n-line peak afso increases relative to the Pt(Sd) peaks for increasing emission angle (0). In addition. the intensities of the sateliite features exhibit interesting behaviour as 3 function of the angle of incidence (cr)_ As can be seen in figs. 2c and 2d, the 4u main-line peak decreases by approximately a factor of two relative to the Safln main-line peak between a! = 0” and cr = 65”_ There is very little, if any, however, change in the intensity of the 17 eV sateliite relative to the intensity of the 14 eV satellite. This effectively eliminates the possibility that these two satellite features simply correspond to the unscreened Su/ln and 40 excitations shifted down in apparent binding energies reiative to the respective screened main-line excitations as is thought to be the case for CO on Cu { 100)
Pt (001) + 4L co
i i f
II
i: i .i
J
I
I
t
I
E,
5
IO
15
BlNDtNG
f 20 ENERGY/
. 25
I
I
30
35
eV
15~. 2. I’liotoelectron spectra for 3 L of CO on Pt (001) at a phoron energy (hv) of 100 eV as a function of emission angle: (a) 0 = O”; and (b) 0 = 60° at n fised angle of incidence (a) of 20”: .md JS d function of anglr of incidence: (c) cy = O”: and (d) cx = 65O for a fixed emission angle (0) of 60p. The arrows marli the approsirnrtte positions of the s.ttelIitc features.
[91Despite the fact that the shake-up features reported for CO on Pt (11 I) f7,SJ and those observed in this study are both apparently associated with core-level excitations, there is one significant difference between the two sets of satellite features. Although the shake-up features associated with the C(ls) excitation were clearly evident only over a few electron-volt photon energy range corresponding to the C(ls) to 2n* transition in CO. the intensity of the satellites observed in this study did not change dm~~atically frrom a few electron volts up to 60 eV above threshold. One possible explanation for this threshold behaviour is that the electron in the Pt(4f) level can be excited into a continuum of unoccupied states in the metal for photon energies above Irv, _ One might be able to gain some insight into the problem of multieIectron excitations by making comparisons with calculations and measurements of shakeup features on metal carbonyls. Such comparisons should be made with some caution, however, in view of the recent study of CO Auger lineshapes 1133 that have shown significant differences in the screening processes which occur for chemisorbed CO compared with those for metal carbonyls. This could be especially important in shake-up phenomena involving chargetransfer excitations from the metal to the adsorbed CO molecule. Based on calculations of metal carbonyls, Loubriel
Volume 100, number 4
CHEhllCAL PHYSICS LETTERS
et al. [7,8] have proposed that charge-transfer excitations are responsible for some of the satellite features observed for CO on Pt( 111) at photon energies near the C(1 s) threshold. These include la-l 5d-1 2s* 1 and 50-l 5d-1 2n’ 1 final states which have corresponding one-electron binding energies of 12 and 17 eV respectively_ These shake-up states result from a 5d to 25r* transition during the C(ls) photoexcitation followed by an Auger decay which leaves the Pt-CO complex in one of the two-hole-one-electron states mentioned above.
16 September 1983
hole in the metal gives rise to several excited states, the relative importance of which depends on the overlap of the frozen configuration of the neutral system with the core electron removed and the relevant ionic states [6]. One such state shown in fig_ 3b would leave an excited electron in the continuum of unoccupied levels in the metal and would also involve a 50 to 5d’ transition_ It is reasonable to expect that such a shake-up excitation
A similar process could be responsible for the ob-
would be important due to the significant amount-of metal character in the 50 level arising from hybridization_ The subsequent decay of the Pt(4f) core hole and the excited electron in the metal via a direct recombina-
served shake-up features that appear at photon energies above the Pt(4f) threshold as shown schematically in fig. 3. In this case, however, one might reasonably expect ligand-to-metal charge-transfer excitations to be
tion mechanism as shown in fig. 3c would result in the Auger-like emission of an electron from a predominantly CO valence level (e.g., 50) or from occupied metal valence levels. Such a direct recombination mechanism
important
has recently been proposed by Bertel et al. [ 141 as the final step in resonant photoemission from the 3d va-
shake-up channels_ The creation
of a core
lence levels of Ti and TiO,. A more detailed study is currently underway to determine if such a mechanism involving charge-transfer excitations is responsible for the observed shake-up features. In addition to the obvious implications of such shake-up phenomena for a better understanding of photoemission itself, there could be important ramifications for photon-stimulated desorption (PSD). Recent studies have shown that structure in the ion yield for PSD from covalently bonded systems can be correlated with metal core-level excitation thresholds fl5,16]. There are also indications that multielectron excitations are often very important in PSD from such systems [ 17,18]_ The creation of a two-hole-one-electron state in levels involved in bonding of CO to the metal
could result in an enhanced PSD cross section. In conclusion, we have shown that the onset of shake-up features in the valence region of CO on Pt{lOO} is correlated with the Pt(4f) excitation threshold. We have proposed that these features correspond to a charge transfer from the metal to empty CO valence Fig. 3. Schematic representation of the proposed shake-up mechanism involvingthe Pt(4f) excitation including: (a) ground state of CO on Pt (001); (b) excitation of an electron from the Pt(4f) core level into unoccupied metal levels (urn) involving a 50 to 5d’ shake-up transition; and (c) direct recombination of the electron in the unoccupied metal levels (urn) with the Pt(4f) hole along with interatomic Auger excitation of an electron from the 50 level resulting in a 5Kz 5d” final state. Note that the degeneracy of each level is not shown explicitly_
levels during a multielectron excitation involving the Pt(4f) level. The subsequent decay of this core hole via a direct recombination mechanism leaves the substrate/ adsorbate complex in a two-hole-one-electron final state. We would like to thank Colin Barnes for his help in preparing the Pt sample, Howard Padmore and the rest of the staff at the Daresbury Synchrotron Radiation 323
Volume
100. number 4
CHEMICAL
PHYSICS LETTERS
Source for technical assistance, and Guillermo Loubriel for useful discussions. In addition, we would like to thank the SERC for a Research Fellowship (DEG) and a Studentship (KGP).
References [l]
0. Gunnarsson and K. Schiinhammer. Phys. Rev. Letters 41 (1978) 160% [ 21 J.C. Fu~~lc, E. Umbach. D. Xlenzel, K. Wandelt and CR. Brundle, Solid State Commun. 27 (1978) 65. [ 3 1 E. Umbach, J .C. Fu,n+c and D. Menzel, J. Electron Spectry. 10 (1977) 15. [-I] P.R. Sorton, R.C. Tapping and Jr.\\‘.Goodale, Surface Sci. 71(197S) 33. IS] 13J. Frcund and E.\V_Plummer. Phys. Rev. B23 (1981) 4859. [6] C.L. Al&n. T. Gustafsson and E.\V. Plummer, Solid State Commun. 24 (1977) 531. [7 ] G. Loubriel, T. Gustafsson, L-1. Johansson and S.J. Ho. Phys. Rev. Lcttcrs49 (1982) 571.
16 September
1983
[8] G.&l. Loubriel and D-R. Jennison, J_ Vacuum Sci. Technol. 20 (1982) 901. [9] C. hlariani, H.U. Middelman, hi. Iwan and K. Horn, to be published. [IO] J.N. Miller, D-T. Ling, PM. Stefan, D.L. Weismann, M.L. Shek, 1. Lindau and W.E. Spicer, Phys. Rev. B24 (1981) 1917. [ 1 I] MA. van Hove, RJ. Koestner, PC. Stab, J_P. Biberian, 1.1. Kesmodel, I. Bartos and G-4. Somorjai. Surface Sci. 103 (1981) 218. [ 121 D.E. Grider, K. Purcell and N-V. Richardson, to be published_ [ 131 B.E. Keel, 3.51. White and G.hl. Loubriel, J. Chem. Phys. 77 (1982) 2665. [ 14) E. Bertel, R. Stockbauer and T.E. hladey, to be published. [ 151 R. Jaeger, R. Treichler and J. Stohr, Surface Sci. 117
(1982) 533. [ 161 R. Stockbauer,
D-M. Hanson, S. Flodstriim and T-E. Xladey, Phys. Rev. B26 (1982) 1885. [ 171 R. Jaeger, J. St&r, R. Treichler and K. Baberschke, Phys. Rev. Letters 47 (1981) 1300. [ 181 T.E. Madey, R. Stockbauer, S-A. Flodstriim, J.F. van der Veen, F-l. Himpsel and DE. Eastman, Phys. Rev. B23 (1981) 6847.
Volumr 1 cl@, number 4
A STUDY OF VALENCE
BAND SHAKE-UP
USING SYNCHROTRON
RADIATION
16 September 1983
PI1YSICS LETTERS
FEATURES
OF CO ON Pt
{loo}
D.E. GRIDER. KG. PURCELL and N.V. RICHARDSON i%e Donnan Laboratories. Um+ersit_v of Liverpool, Grove Street. PO Box 147. Liverpool L69 3BX. UK Recribed 11 April 1983: in final form Z July 1983
A stud! of the shAe-up fearures in rhc valence region of CO on Pt {loo) is presented. The onset of these features is shown IO be correlntcd wirh the excitation threshold for the I’t(4t-l levels. A possible mechanism for the shake-up process is proposed in uhich the Pt(Jfl hole results in exited skxtes other than a single electron-hole state. Subsequently, the Pt(lf) hole decals via a direct recombination process resultins in the observed satellite features. The implications of such a shakeup process for photon-srirnulatcd desorprion (PSD) are also discussed.
We report the first clear evidence for interatomic shake-up features associated with the valence levels of a cbemisorbed molecule_ The onset of shake-up structures in the valence region of CO/Pt { 100) is shown to be correlated with the Pt(4f) excitation threshold_ Such an onset indicates that the presence of this core-hole results in intermediate escited states other than a single electron-hole state. Subsequent decay of these states i;lvolving a direct recombination mechanism for the Pr(4f) hole results in the observed satellite features_ The investigation of satellite structures arising from photoexcitation
of electrons
from
core
levels has long
been an area
of intense interest in both gas-phase and surface-adsorbate studies [l-6] _ Recently, however, there has been increased interest in similar satellites asso&ted with valence levels or adsorbed molecules [6-IO]_ In a recent paper. Loubriel et al. [ 71, have observed valence shake-up states associated with the creation of a hole in the C( 1s) level of CO on Pt {I 11). These shake-up features were observable only over a very narrow photon energy range corresponding to the resonant photoescitation of an electron level of CO followed by 311 Auger decay process. The predominant features were associated with local ligand excitations (i.e. 5~~’ lsr-l 2~*l, 50-z %‘I, ln-2 %‘I. and 40-1 50-1 25r*1) as might be expected due to the presence of the C(ls) hole on
the CO molecule
itself. In addition,
however, less
intense lower binding energy satellites here observed which were associated with charge-transfer excitations (In-’ 5d-’ 2n*‘, and 5u-’ 5d-’ 2~*~). On the basis of this work, one might reasonably expect to observe analogous valence band satellite features upon creation of a core hole in the metal substrate followed by an Auger decay process. Indeed, recent photoemission studies of CO on Pt [lo] show a weak CO-induced shoulder =5 eV below the 40 peak over a photon energy range from 120 eV up to 190 eV. This feature, however. was tentatively ascribed to a 5d to 2n* excitation based on comparison with shake-up structures in the valence bands of transition-metal carbonyls [5,6] _ These structures are thought to correspond to screening of 4u and 50 valence holes by charge transfer of metal electrons to the In* orbital as well as local ligand excitations. Recent studies of CO on CU { 100) [9] have shown that screening of valence holes on CO by metal electrons can result in striking satellite features just as it does for core holes [4]. Strong satellites are observed ~1.7 eV below the 40 and %/In main-line peaks. These features were attributed to final-state configurations in which the 4u,5u. and 1s valence holes are screened (main-line peaks) or relatively unscreened (satellites) by the metal electrons. In addition, there is a very weak feature a5.5 eV below the 40 main-line peak which is believed to be due to intramolecular ligand excitations (i.e. Ix to 2n*).
320 0 009~2614/83/0000-0000/S
03.00 G 1983 North-Holland
Volume 100. number 4
CHEMICAL
PHYSICS LETTERS
16 September
1983
The purpose of this work is to find evidence for valence band shake-up features associated with the creation of a core hole in the metal substrate_ It should be possible to distinguish between such features and those due to screening of valence holes in the adsorbate by metal electrons or due to local ligand excitations simply by Iooking for the onset of satellites at a photon energy corresponding to the creation of a core hole in the metal. An estimate of the photon energy threshold of such an excitation involving a core hole of ionization potential (1r,) is given by hu,
=‘n
-am
+A,,
(1)
where @nr is the work function of the metal and As is the energy given over to the shake-up state as determined by the difference in the apparent binding energies between the CO main-line and shake-up peaks. This results in Jzvth =72+3eV
(2)
for a Pt(4f) excitation threshold and a shake-up energy (As) of 6 i- 3 eV_ The measurements were carried out at the Daresbury Synchrotron Radiation Source using a Vacuum Generator ADES_400 photoelectron spectrometer_ Photoelectrons were collected along the [ 1001 plane of incidence for a range of emission angles (8) and various angles of incidence (or) for p-polarized light. Following repeated argon-ion bombardment/anneal cycles, the surface cleanliness of the Pt {I 00) crystal was monitored using AES. In addition, a clear (5 X 20) LEED pattern was observed, indicative of the cIean reconstructed Pt { 100) surface 11 I] _For all of the measurements presented here, the sample was dosed with 4 L of CO at 300 K. The background pressure during the measurements was in the low 1O-lo mbar range. As shown in fig. la the photoemission spectra (Jzv= 65 eV) at photon energies below the threshold for the creation of a Pt(4f) core hole exhibits the normal COinduced photoemission peaks at binding energies of 8.8 eV (%/lrr), 11.3 eV (4~) and 32.0 eV (3~). In addition, there is a weak OW Auger feature at an apparent binding energy of 23 eV_ At a photon energy of 7 1 eV, as shown in fig. I b, additional CO-related features appear at one-electron binding energies of -14 and 17 eV. At a slightly higher photon ener,T of 77 eV, as shown in fig. 1c, the 17 eV feature is well resolved and there is still a broad shoulder in the region of 14 eV binding energy.
J, ?I$
1
5
1-
10
3tNDlNG
I
15
I
20
I
25
I
30
I
35
ENERGY/eV
Fig_ 1. Photoelectron spectra for 4 l_ of CO on IQ {OOI) at an ate of incidence (&I of 40’ and emission an& (6) of 35O as a function of photon energy: (a) hv = 65 eV; (b) hu = 71 eV; and (c) hv = 77 eV. The arrows mark the approximate positions of the satellite features. In curve (c), the feature aboveEfis due to excitation of the Pt(4f) levels by second-order light from the monochromator_
The onset of the 17 eV satellite structure at a photon energy of 71 eV is in excellent agreement with the estimate for JzvU, given by eq. (3) Although the 14 eV shoulder is not well resolved at all photon energies, it too has a threshold in the same photon energy region. This is a clear indication that both of these shake-up features are associated with the creation of a Pt(4f) core hole. Both of these structures remain at the same binding ener,v and have approximately the same intensities relative to the Su/ls and 40 main-line peaks up to a photon ener7 of at least 130 eV. As noted previously, similar structures have been observed up to =I90 eV
WIAlthough
a detailed
discussion
of the dependence
of
321
CIiE!MC_M_ PHYSICS LETTERS
Solurnc 1130.number 4
the sateIiire features on angle of incidence (CY)of the llgbt and photoelectron emission angle (0) will be presented in a later work 1 I?]. some general trends should be noted. As might be expected simply on the basis of electron escape depth arzunrents. both the 14 and 17 eV satellites increase as one moves away from normal emission (0 = 0). Xs shown in figs. ta and 2b. the dramatic increase in the 40 main-line feature (1 1.3 eV) between 0 = 0” and 0 = 60” for constant angle of in-
16
September 1983
cidence (a = 20”) is mirrored in both the 14 and 17 eV satellite structures. It is not, however, possible to eliiinate completely the possibility that excitations of 50 or 1s levels are also involved in these satellite structures since the So/lx n&n-line peak afso increases relative to the Pt(Sd) peaks for increasing emission angle (0). In addition. the intensities of the sateliite features exhibit interesting behaviour as 3 function of the angle of incidence (cr)_ As can be seen in figs. 2c and 2d, the 4u main-line peak decreases by approximately a factor of two relative to the Safln main-line peak between a! = 0” and cr = 65”_ There is very little, if any, however, change in the intensity of the 17 eV sateliite relative to the intensity of the 14 eV satellite. This effectively eliminates the possibility that these two satellite features simply correspond to the unscreened Su/ln and 40 excitations shifted down in apparent binding energies reiative to the respective screened main-line excitations as is thought to be the case for CO on Cu { 100)
Pt (001) + 4L co
i i f
II
i: i .i
J
I
I
t
I
E,
5
IO
15
BlNDtNG
f 20 ENERGY/
. 25
I
I
30
35
eV
15~. 2. I’liotoelectron spectra for 3 L of CO on Pt (001) at a phoron energy (hv) of 100 eV as a function of emission angle: (a) 0 = O”; and (b) 0 = 60° at n fised angle of incidence (a) of 20”: .md JS d function of anglr of incidence: (c) cy = O”: and (d) cx = 65O for a fixed emission angle (0) of 60p. The arrows marli the approsirnrtte positions of the s.ttelIitc features.
[91Despite the fact that the shake-up features reported for CO on Pt (11 I) f7,SJ and those observed in this study are both apparently associated with core-level excitations, there is one significant difference between the two sets of satellite features. Although the shake-up features associated with the C(ls) excitation were clearly evident only over a few electron-volt photon energy range corresponding to the C(ls) to 2n* transition in CO. the intensity of the satellites observed in this study did not change dm~~atically frrom a few electron volts up to 60 eV above threshold. One possible explanation for this threshold behaviour is that the electron in the Pt(4f) level can be excited into a continuum of unoccupied states in the metal for photon energies above Irv, _ One might be able to gain some insight into the problem of multieIectron excitations by making comparisons with calculations and measurements of shakeup features on metal carbonyls. Such comparisons should be made with some caution, however, in view of the recent study of CO Auger lineshapes 1133 that have shown significant differences in the screening processes which occur for chemisorbed CO compared with those for metal carbonyls. This could be especially important in shake-up phenomena involving chargetransfer excitations from the metal to the adsorbed CO molecule. Based on calculations of metal carbonyls, Loubriel
Volume 100, number 4
CHEhllCAL PHYSICS LETTERS
et al. [7,8] have proposed that charge-transfer excitations are responsible for some of the satellite features observed for CO on Pt( 111) at photon energies near the C(1 s) threshold. These include la-l 5d-1 2s* 1 and 50-l 5d-1 2n’ 1 final states which have corresponding one-electron binding energies of 12 and 17 eV respectively_ These shake-up states result from a 5d to 25r* transition during the C(ls) photoexcitation followed by an Auger decay which leaves the Pt-CO complex in one of the two-hole-one-electron states mentioned above.
16 September 1983
hole in the metal gives rise to several excited states, the relative importance of which depends on the overlap of the frozen configuration of the neutral system with the core electron removed and the relevant ionic states [6]. One such state shown in fig_ 3b would leave an excited electron in the continuum of unoccupied levels in the metal and would also involve a 50 to 5d’ transition_ It is reasonable to expect that such a shake-up excitation
A similar process could be responsible for the ob-
would be important due to the significant amount-of metal character in the 50 level arising from hybridization_ The subsequent decay of the Pt(4f) core hole and the excited electron in the metal via a direct recombina-
served shake-up features that appear at photon energies above the Pt(4f) threshold as shown schematically in fig. 3. In this case, however, one might reasonably expect ligand-to-metal charge-transfer excitations to be
tion mechanism as shown in fig. 3c would result in the Auger-like emission of an electron from a predominantly CO valence level (e.g., 50) or from occupied metal valence levels. Such a direct recombination mechanism
important
has recently been proposed by Bertel et al. [ 141 as the final step in resonant photoemission from the 3d va-
shake-up channels_ The creation
of a core
lence levels of Ti and TiO,. A more detailed study is currently underway to determine if such a mechanism involving charge-transfer excitations is responsible for the observed shake-up features. In addition to the obvious implications of such shake-up phenomena for a better understanding of photoemission itself, there could be important ramifications for photon-stimulated desorption (PSD). Recent studies have shown that structure in the ion yield for PSD from covalently bonded systems can be correlated with metal core-level excitation thresholds fl5,16]. There are also indications that multielectron excitations are often very important in PSD from such systems [ 17,18]_ The creation of a two-hole-one-electron state in levels involved in bonding of CO to the metal
could result in an enhanced PSD cross section. In conclusion, we have shown that the onset of shake-up features in the valence region of CO on Pt{lOO} is correlated with the Pt(4f) excitation threshold. We have proposed that these features correspond to a charge transfer from the metal to empty CO valence Fig. 3. Schematic representation of the proposed shake-up mechanism involvingthe Pt(4f) excitation including: (a) ground state of CO on Pt (001); (b) excitation of an electron from the Pt(4f) core level into unoccupied metal levels (urn) involving a 50 to 5d’ shake-up transition; and (c) direct recombination of the electron in the unoccupied metal levels (urn) with the Pt(4f) hole along with interatomic Auger excitation of an electron from the 50 level resulting in a 5Kz 5d” final state. Note that the degeneracy of each level is not shown explicitly_
levels during a multielectron excitation involving the Pt(4f) level. The subsequent decay of this core hole via a direct recombination mechanism leaves the substrate/ adsorbate complex in a two-hole-one-electron final state. We would like to thank Colin Barnes for his help in preparing the Pt sample, Howard Padmore and the rest of the staff at the Daresbury Synchrotron Radiation 323
Volume
100. number 4
CHEMICAL
PHYSICS LETTERS
Source for technical assistance, and Guillermo Loubriel for useful discussions. In addition, we would like to thank the SERC for a Research Fellowship (DEG) and a Studentship (KGP).
References [l]
0. Gunnarsson and K. Schiinhammer. Phys. Rev. Letters 41 (1978) 160% [ 21 J.C. Fu~~lc, E. Umbach. D. Xlenzel, K. Wandelt and CR. Brundle, Solid State Commun. 27 (1978) 65. [ 3 1 E. Umbach, J .C. Fu,n+c and D. Menzel, J. Electron Spectry. 10 (1977) 15. [-I] P.R. Sorton, R.C. Tapping and Jr.\\‘.Goodale, Surface Sci. 71(197S) 33. IS] 13J. Frcund and E.\V_Plummer. Phys. Rev. B23 (1981) 4859. [6] C.L. Al&n. T. Gustafsson and E.\V. Plummer, Solid State Commun. 24 (1977) 531. [7 ] G. Loubriel, T. Gustafsson, L-1. Johansson and S.J. Ho. Phys. Rev. Lcttcrs49 (1982) 571.
16 September
1983
[8] G.&l. Loubriel and D-R. Jennison, J_ Vacuum Sci. Technol. 20 (1982) 901. [9] C. hlariani, H.U. Middelman, hi. Iwan and K. Horn, to be published. [IO] J.N. Miller, D-T. Ling, PM. Stefan, D.L. Weismann, M.L. Shek, 1. Lindau and W.E. Spicer, Phys. Rev. B24 (1981) 1917. [ 1 I] MA. van Hove, RJ. Koestner, PC. Stab, J_P. Biberian, 1.1. Kesmodel, I. Bartos and G-4. Somorjai. Surface Sci. 103 (1981) 218. [ 121 D.E. Grider, K. Purcell and N-V. Richardson, to be published_ [ 131 B.E. Keel, 3.51. White and G.hl. Loubriel, J. Chem. Phys. 77 (1982) 2665. [ 14) E. Bertel, R. Stockbauer and T.E. hladey, to be published. [ 151 R. Jaeger, R. Treichler and J. Stohr, Surface Sci. 117
(1982) 533. [ 161 R. Stockbauer,
D-M. Hanson, S. Flodstriim and T-E. Xladey, Phys. Rev. B26 (1982) 1885. [ 171 R. Jaeger, J. St&r, R. Treichler and K. Baberschke, Phys. Rev. Letters 47 (1981) 1300. [ 181 T.E. Madey, R. Stockbauer, S-A. Flodstriim, J.F. van der Veen, F-l. Himpsel and DE. Eastman, Phys. Rev. B23 (1981) 6847.